In recent years, semiconductor memories utilizing resistance variation elements as storage elements, such as phase-change random access memories (PRAMs) and magnetic random access memories (MRAMs), have received attention and have been developed. The MRAMs are devices in which memory cells are made to store information “1” or “0”, using magnetoresistive effects, to thereby perform a memory operation, and are featured in that they have nonvolatility, a high-speed operation, a high integration and a high reliability.
A large number of MRAMs have been reported which use elements exhibiting a tunneling magnetoresistive (TMR) effect which is one of the above magnetoresistive effects. As a magnetoresistive element, a magnetic tunnel junction (MTJ) element is provided which includes a three-layer lamination structure having a storage layer having a variable magnetization direction, an insulating film serving as a tunnel barrier and a reference layer maintaining a predetermined magnetization direction.
A resistance of the MTJ element varies in accordance with the magnetization directions of the storage layer and the reference layer, it takes a low resistance state when the magnetization directions are parallel, and it takes a high resistance state when the magnetization directions are antiparallel; and information is stored by associating the above parallel and antiparallel states with binary information “0” and “1”.
Methods of writing information to the MTJ element include a magnetic-field writing method in which only the magnetization direction of the storage layer is reversed by current magnetic field that is generated when current is flowed through a writing line, and a writing (current driven switching) method using a spin angular momentum transfer in which the magnetization direction of the storage layer is reversed by passing a spin polarized current through the MTJ element itself.
In the former method, when the element size is reduced, a coercive force of a magnetic substance constituting the storage layer increases, and the write current tends to be increased, and thus it is hard to achieve both miniaturization of the element and lowering of the current at the same time.
On the other hand, in the latter method, the number of spin polarized electrons to be injected decreases as the volume of a magnetic substance constituting the storage layer decreases, it is expected that both miniaturization of the element and lowering of the current will be easily achieved at the same time.